1. Field of the Disclosure
The present disclosure related to an apparatus and a method for controlling an inverter. More specifically, the present disclosure relates to an apparatus and a method for controlling an inverter to measure each phase current using a leg-shunt resistor.
2. Discussion of the Related Art
In general, an inverter is a device converting direct current (DC) into alternating current (AC) electrically. The inverter controls speed of a motor by receiving electricity from a commercial power supply, and by altering voltage and frequency in the inverter to supply to the motor.
FIG. 1 is a block diagram of a general inverter system.
As illustrated in the figure, the inverter (1) drives the motor (2) whereby the rectification unit (10) converts a 3-phase electricity inputted to DC electricity, and the DC-link capacitor (20) accumulates the DC electricity, and then the inverter unit (30) converts the accumulated DC electricity to AC electricity again and alters voltage and frequency. Therefore, the inverter is also referred to as a variable voltage variable frequency (VVVF) system.
Recently, a current detection method using shunt resistors is commonly used in small inverters for the purpose of acquiring cost competitiveness. The current detection method using shunt resistor may be categorized into a DC-link shunt resistor current detection method, an output phase shunt resistor current detection method, and a leg-shunt resistor current detection method, according to positions of the shunt resistor.
FIG. 2 is a block diagram illustrating a leg-shunt resistor current detection method.
As illustrated in the figure, the leg-shunt resistor current detection method is a method where a shunt resistor is arranged at an emitter terminal of the lower insulated gate bipolar transistor (IGBT) (22) in the inverter unit (30) to detect currents, which has advantages of realizing a circuit at a low cost and detecting an instantaneous current as well.
However, the method using the leg-shunt resistor also has a problem that the current detection area is restricted by pulse width modulation (PWM) switching status of the IGBT.
FIG. 3 is an exemplary view illustrating a current detection restricted area of a leg-shunt-type inverter. FIG. 4 is an exemplary view illustrating a phase current detection section in a leg-shunt-type inverter.
The space vector PWM (SVPWM) is formed of six non-zero vectors and two zero vectors. The PWM control unit transforms the 3-phase output current of the inverter into a low-voltage reference vector V* on a two-dimensional plane between d-axis and q-axis. The V* is formed of a combination of two adjacent non-zero vector and a zero vector.
As illustrated in FIG. 4, in Sector 1 (referring to FIG. 3), during the first half of the PWM cycle, switching vectors are in sequential order of a zero vector V0 (0, 0, 0), a non-zero vector 1 V1 (1, 0, 0), a non-zero vector 2 V2 (1, 1, 0), and a zero vector V7 (1, 1, 1), then during the last half of the PWM cycle, the switching vectors are applied in reverse order (it is referred to as ‘symmetric SVPWM’).
In the structure as illustrated in FIG. 2, current detection of each phase by the leg-shunt-type inverter is practicable when lower IGBTs of each phase are turned-on for the current to flow to the shunt resistors (23). In addition, current detection of the inverter 3 phases is practicable in the section where at least two of the lower IGBTs are turned-on in condition that 3 phases of the inverter are in parallel, as illustrated in FIG. 4.
When two IGBTs are turned-on so that current detection of 2 phases becomes practicable, the rest of one phase current may be detected indirectly through calculating the relation formula of ius+ivs+iws=0. The result may differ by sectors categorized in FIG. 3, as described in the following TABLE 1.
TABLE 1SectorIuIvIw1Iu = (Ivs + Iws)Iv = −IvsIw = −Iws2Iu = −IusIv = (Ius + Iws)Iw = −Iws3Iu = −IusIv = (Ius + Iws)Iw = −Iws4Iu = −IusIv = −IvsIw = (Ius + Ivs)5Iu = −IusIv = −IvsIw = (Ius + Ivs)6Iu = (Ivs + Iws)Iv = −IvsIw = −Iws
For the purpose of controlling the vectors to escape from the current detection restricted area in such the leg-shunt-type inverter, there is a method for controlling a voltage reference vector to escape from the current detection restricted area by altering magnitude and angle of the voltage reference vector when the voltage reference vector enters the current detection restricted area. The method is according to the following EQUATION 1.Tsamp_min=tdt+trs+2tsn  EQUATION 1
where tdt is an inverter dead-time; trs is a current detection circuit delay time; tsn is an AD converter sampling time; and tsamp_min is a minimum detection time of the leg-shunt resistor current detection.
However, the conventional technology described in the above is not considering a method to cope with a situation where the current detection is not performed precisely outside of the expected current detection unavailable area.
Meanwhile, in the inverter which is vector-controlled without sensor, a rotor flux is estimated as illustrated in FIG. 5, and the rotating velocity of a rotor is estimated based on the rotor flux without a separate velocity detector such as an encoder. FIG. 5 is an exemplary view illustrating t rotor flux estimation unit.
As illustrated in FIG. 5, when the rotating velocity of the rotor is estimated by detecting the rotor flux, when the measured current is considerably different even instantaneously from the actual physical current, there occurs a possibility where a number of sensorless vector control modules including the rotor flux estimation unit are operating unstably.
                                          w            sl                    =                                    1                              T                r                                      ⁢                                          i                qs                e                                            i                ds                e                                                    ⁢                                  ⁢                              θ            e                    =                      ∫                                          (                                                      w                    r                                    +                                      w                    sl                                                  )                            ⁢                              ⅆ                t                                                    ⁢                                  ⁢                              T            r                    =                                    L              r                        /                          R              r                                                          EQUATION        ⁢                                  ⁢        2            
The method for indirect-vector-controlling an induction motor without sensor is to calculate a synchronous angle θe to use in calculation of a reference voltage vector as described in EQUATION 2. To this end, the rotor velocity values of wr and wsl are required to be precisely evaluated.
The rotor velocity wr is estimated by the estimated rotor flux, and the slip velocity wsl is proportional to the proportion of q-axis current to d-axis current, as described in EQUATION 1.
In a leg-shunt-type inverter, it is experimentally ascertained that there is a very high probability to read current information incompletely around the (0 1 1), (1 0 1), (1 1 0) vector boundaries where the sectors are intersecting on the output voltage vector diagram illustrated in FIG. 3, even outside of the current detection unavailable area defined in EQUATION 1.
FIG. 6 is an experimental waveform illustrating observation of an instability phenomenon of inverter sensorless control when limit performance of a leg-shunt resistor current detection circuit is not complemented. FIG. 7 is an experimental waveform illustrating a low-pass filtered current which is detected by a leg-shunt-type inverter.
Referring to FIG. 6, because a predetermined load is on operation, a pulsation should not occur at iqse. In addition, referring to FIG. 7, the waveform on the left is a result of coordinate transformation of 3-phase current into 2-phase current, where the yellow waveform is d-axis stator current; the red waveform is q-axis stator current; and the green waveform is U-phase current measured by an oscilloscope for comparison. The waveform on the right is a result of phase plotting of stator current vectors on the left.
That is, it is experimentally ascertained that there is a probability to read current information incompletely, not only inside of the triangles posed around the vector vertices, but also in areas around the boundary lines of relevant vectors where zero vector time seems to be secured sufficiently.
Such a problem may be determined as not considerably affecting magnitude of the estimated flux, not because the current considerably different from the actual current detected by the leg-shunt is continuously detected, but because values multiplied by sampling time during a short period are integrated in the flux estimation unit integration when the current is detected in the amount of around one sample.
However, it is experimentally observed a phenomenon where magnitude and angle of the current vector (iqse) instantaneously jump at the moment of observing an abnormality current value as illustrated in FIG. 6, when applying more than a certain load (around 100% load of rated load) to the motor controlled by the inverter and phase plotting on the d-q axis coordinate plane of stator current vector on the stationary reference frame. And in the next step, a phenomenon of returning in retrograde to values of the existing vectors is observed.
Furthermore, in this situation, vs−Rs·is, which is a difference between stator voltage and stator voltage drop across a stator resistance operating as an integration object in the stator flux estimation unit, has an instantaneously high peak value. Here, in order to perform this calculation, when variables are set in comparatively wide areas to increase calculative resolution in middle-low price micro-controller unit (MCU)s and digital signal processor (DSP)s which are only dealing with integer operations, a variable over-flow may occur and thus the variable is initialized to cause step variation of the estimated flux. After then, the flux estimation is not performed properly. This may cause problems such as inverter out-of-step phenomenon, etc.
In addition, referring to FIG. 6, it may be observed that the d-axis element of stator current on the synchronous reference frame is repeatedly showing step variations in spite of constant reference. This phenomenon may generate errors in calculation of slip velocity as described in the first equation of EQUATION 2, and may incur adverse effects such as estimation velocity error to sensorless control performance of indirect vector control type of the induction motor which is seriously affecting efficiency of synchronous angle calculation.